Metal strips, for example steel strips but also aluminium or aluminium alloy strips, are conventionally subjected to heat treatments in order to provide specific properties of the metal strip. For this purpose, steel strips, but also aluminium alloy strips, are heated and/or very quickly cooled using temperature-control means. However, the temperature ranges in which steel strips and aluminium alloy strips are heated in heat treatments differ considerably. Heating steel strips to achieve recrystallisation lies in the range of 950° C. and above, whereas aluminium alloy strips already recrystallise at temperatures of around 300° C. In the case of some aluminium alloys, however, for example for subsequent precipitation hardening, alloy elements must be put into solution, in which temperatures of 580° C. must be reached. In order to subsequently be able to keep the alloy elements in an oversaturated state in the matrix, it is necessary to quickly quench from this temperature.
Often, high heating and cooling rates are also required for other metallurgical reasons (e.g. for grain refinement). Due to the need for high heating or cooling speeds, heat treatments of this type cannot be carried out on the wound-up coil, but rather must take place on the continuous strip in what are known as continuous furnaces and cooling lines. The rapid heating or cooling causes thermal stresses, which, in particular in the case of thin strips, lead to distortions, which can both prevent a steady strip run during the ongoing process and cause flatness defects in the finished product.
The objective of a heat treatment on the continuous strip is to change the temperature level homogeneously over the entire width of the strip in a short space of time in order to change the properties uniformly over the entire width of the strip in a desired manner. In the case of a uniform change in temperature taking place linearly and transversely to the strip, however, thermally induced transverse stresses always occur, which cause distortions. The reason for this is that strip fibres close to the centre are constricted under thermal loading by the neighbouring fibres in the transverse flow, whereas the strip edges can expand and contract freely.
The prior art discloses, both for cooling metal strips made of steel and for cooling aluminium strips, devices for changing the temperature of the metal strip, comprising means for cooling the metal strip and means for conveying the metal strip relative to the means for changing temperature, by which a metal strip can be for example continuously cooled. Such a method, in which both temperature and flatness are measured on a thick steel strip, is disclosed in the European Patent Application EP 1 634 657 1. The International Patent Application WO 2009/024644 A1 also relates to a method and to a device for checking the flatness of steel strips, which comprises individually activatable temperature-control means arranged at fixed distances for bringing about specific cooling of the steel strip. A fixed arrangement of the cooling means for cooling a metal strip made of an aluminium alloy is disclosed by the US Patent Application US 2014/0250963 A1. Despite the cooling power of the individual temperature-control means being controlled according to the flatness of the metal strip after cooling or according to temperature measurements of the metal strip after cooling, the existing concepts for changing the temperature of a metal strip, that is to say the concepts for heating metal strips for heat treatment and the concepts for cooling metal strips after a heat treatment are in need of improvement, since there are still problems relating to flatness defects in production.
The invention therefore addresses the object of providing a device for changing the temperature of metal strips, which, in particular in the case of changes in temperature of aluminium strips, allows improved process control and greater precision in terms of the flatness of the treated metal strip. Furthermore, a preferred use of the device according to the invention and a method for changing the temperature of a metal strip using the device according to the invention are to be proposed.